1. Field of the Invention
The present invention relates to a sheet transfer mechanism for a printer to form an image on a sheet being transferred in a transfer route by propelling ink at an image former.
2. Description of Related Arts
When an inkjet type printer forms an image by propelling ink droplets on a sheet, it is necessary to keep a certain space between a surface of the sheet and ink heads in order to avoid influences of wrinkles and a wavelike deformation (hereinafter referred to as cockling) as much as possible, so that stable printing is performed. In addition, when a sheet gets swelling due to water-based ink or moisture, it is necessary to prevent the sheet on a transfer belt from being lifted toward the print head.
Therefore, such an inkjet type printer requires a method for tightly attaching a sheet to a transfer mechanism. To do so, it has been well known a printer capable of transferring a sheet and attaching the sheet to the transfer mechanism evenly and strongly.
However, it is actually difficult to transfer a sheet keeping in an even condition, while attaching and pressing the sheet to the transfer mechanism. The more a printer is required to prevent a sheet from cockling widely, the more cockling is caused locally. As a result, local cockling portions of the sheet may come into contact with ink heads.
Meanwhile, Patent Document 1 (Japanese Patent Laid-Open Publication No. H07-133035) discloses a sheet transfer mechanism with an electrostatic adherence system capable of keeping an uplift of cockling portions of a sheet under a certain height by tightly attaching the sheet to the sheet transfer mechanism by use of a comb-type electrode, which prevents the cockling portions from influencing printing performance.
In addition, Patent Document 2 (Japanese Patent Laid-Open Publication No. 2007-031007) discloses a sheet transfer mechanism with an air suction system. The sheet transfer mechanism has two or more rollers provided with given intervals for driving a circular belt, formed with through-holes, having a larger width than a transfer sheet in a direction perpendicular to a sheet transfer direction. The sheet transfer mechanism suctions air from through-holes provided on the belt in order to bring the through-holes under negative pressure with respect to atmospheric pressure.